Display device

ABSTRACT

Transflective display device provided with an optical waveguide which splits light coming in from aside into two light beams having a mutually opposite circular polarization. Polarization splitting is achieved at the interface of areas in the optical waveguide having a chiral nematic structure.

[0001] The invention relates to a display device comprising an imagedisplay panel having a first substrate which is provided withlight-reflecting electrodes at the area of pixels, an illuminationsystem comprising an optical waveguide of an optically transparentmaterial having an exit face facing the image display panel and aplurality of end faces, at least one of said end faces being an entranceface for light, while light can be coupled into said end face of theoptical waveguide.

[0002] The image display panel may comprise an electro-optical medium(between two substrates) such as liquid crystalline material or anelectrochromic material. It may also be based on electrostatic forces(deformable mirrors).

[0003] The invention also relates to an illumination unit (or frontlight) for such a display device and to methods of manufacturing suchillumination units.

[0004] Such reflective display devices are used in, for example,portable apparatus such as laptop computers, mobile telephones, personalorganizers, etc. With a view to saving energy, it is desirable that thelight source can be switched off in the case of sufficient ambientlight.

[0005] A display device of the type mentioned above is described in WO99/22268 (PHN 16.374). In the optical waveguide described in thisdocument, an unpolarized beam from the light source is split up into twomutually perpendicularly polarized beam components. Polarizationseparation is obtained by causing the unpolarized beam to be incident onan interface between an area of isotropic material having a refractiveindex n_(p) and an area of anisotropic material having refractiveindices n_(o) and n_(e), in which one of the two indices n_(o) or n_(e)is equal or substantially equal to n_(p). When an unpolarized beam isincident on such an interface, the beam component which does notexperience any refractive index difference at the transition betweenisotropic and anisotropic material is passed in an undeflected form,whereas the other beam component is deflected or reflected. One of thetwo beam components is subsequently passed by a polarizer to areflective liquid crystal panel. The optical waveguide shown exhibitsmuch less image distortion than a known optical waveguide with a groovestructure (microprisms) on the viewing side of the optical waveguide.The image distortion is produced because the groove structure hasdifferent slopes, which results in multiple image formation. Generally,this multiple image formation is prevented by providing an opticalcompensator having a complementary groove structure.

[0006] However, in the display device described in WO 99/22268 (PHN16.374), stray light is generated in the viewing direction on theinterface between the areas with isotropic and anisotropic material.

[0007] Moreover, the light which is deflected in the direction of theimage display panel sometimes undergoes partial reflections in the imagedisplay panel and in the optical waveguide before the light reaches thereflecting pixels.

[0008] These drawbacks apply to the same or an even greater extent tooptical waveguides which are based on a groove structure.

[0009] It is, inter alia, an object of the present invention to providea solution to the above-mentioned problem.

[0010] To this end, a display device according to the invention ischaracterized in that the optical waveguide is present between the imagedisplay panel and a circular polarizer, and the optical waveguidecomprises polarizing means for substantially circularly polarizing theentering light. In this application, the word circular is alsounderstood to be “elliptical”. In certain circumstances (when lesscontrast is sufficient) it is also possible to work with ellipticallypolarized light.

[0011] The polarizer may be integrated in the display device.

[0012] The polarizing means have a similar function as in the knowndevice, namely polarizing light rays from the light source, in whichlight of one kind of polarization (for example, levorotatorypolarization) is deflected in the direction of the image display panel.In the relevant case, light exiting on the viewing side (dextrorotatorypolarized light in the same example) is not passed by the polarizer.

[0013] Due to polarization, an unpolarized beam from the light source issplit up into two mutually oppositely polarized beam components(levorotatory and dextrorotatory). Such a polarization separation isobtained, for example, by causing the unpolarized beam to be incident onan interface between an area of isotropic material and an area of chiralnematic material, for example, a chiral nematic liquid crystal materialprovided in, for example, a groove structure, or a (patterned) chiralnematic network. When an unpolarized beam is incident on such aninterface, a beam component of one handedness is passed undeflected onthe transition between isotropic and chiral nematic material, while thebeam component having the other, opposite handedness is deflected orreflected.

[0014] A suitable embodiment is characterized in that the pitch of thechiral nematic liquid crystal material or the chiral nematic polymernetwork within a groove varies. A larger bandwidth of the reflectedlight can thereby be obtained.

[0015] A first method of manufacturing such an illumination unit (orfront light) provided with polarizing means for circularly (orelliptically) polarizing the entering light is characterized in that asurface of a transparent body is provided with grooves, and thetransparent body within the grooves is provided with a chiral nematicliquid crystal material or the chiral nematic polymer network.

[0016] A second method is characterized in that a surface of atransparent body is provided with a layer of a chiral nematic materialwhich is locally converted into isotropic material.

[0017] These and other aspects of the invention are apparent from andwill be elucidated with reference to the embodiments describedhereinafter.

[0018] In the drawings:

[0019]FIG. 1 is a cross-section of an embodiment of a reflective displaydevice according to the invention.

[0020]FIG. 2 is a cross-section of an optical waveguide, while

[0021]FIGS. 3 and 4 are variants of FIG. 2, and

[0022] FIGS. 5 to 7 are cross-sections of an optical waveguide duringone stage of a plurality of possible manufacturing methods.

[0023] The Figures are diagrammatic and not to scale. Correspondingcomponents generally have the same reference numerals.

[0024] The display device 1 shown diagrammatically in FIG. 1 comprisesan image display panel 2 and an illumination system (or front light) 8.

[0025] The image display panel 2 comprises a liquid crystalline material5 between two substrates 3, 4, based on the twisted nematic (TN), thesupertwisted nematic (STN) or the ferroelectric effect so as to modulatethe direction of polarization of incident light. The image display panelcomprises, for example, a matrix of pixels for which light-reflectingpicture electrodes 6 are provided on the substrate 3. The substrate 4 islight-transmissive and has one or more light-transmissive electrodes 7of, for example, ITO (indium tin oxide). The picture electrodes areprovided with electric voltages via connection wires 6′, 7′ which areprovided with drive voltages by means of a drive unit 9. The substratesand electrodes are coated with orientation layers 15 in known manner.

[0026] The illumination system 8 comprises an optical waveguide 18 whichis made of an optically transparent material and has four end faces 10,10′. A light source 12 whose light is coupled into the optical waveguide18 via one of the end faces, for example 10, is situated opposite thisend face. The light source 12 may be, for example, a rod-shapedfluorescence lamp. The light source may alternatively be constituted byone or more light-emitting diodes (LED), notably in flat panel displaydevices having small image display panels such as, for example, portabletelephones. Moreover, the light source 12 may be detachable.

[0027] The exit face 16 of the optical waveguide 8 faces the imagedisplay panel 2. Each end face 10′ of the transparent plate in whichlight is not coupled in may be provided with a reflector. In this way,light which is not coupled out on the exit face 16 and consequentlypropagates through the optical waveguide and arrives at an end face isthus prevented from leaving the optical waveguide 11 via this end face10′.

[0028] To prevent light from leaving the optical waveguide 18 withoutcontributing to the light output of the illumination system, light ofthe lamp 12 is preferably coupled into the optical waveguide 18 viacoupling means 13, for example, by means of a wedge-shaped opticalwaveguide which limits the angle of the entering beam with respect tothe exit faces 16, 17 to, for example, 15 degrees. Moreover, thecontrast is enhanced because there is no stray light.

[0029] A light beam 20 from the lamp 12 is converted in a manner to bedescribed below into circularly polarized light so that mainly light ofone handedness is deflected towards the reflective image display panel 2(beams 21) and, dependent on the state of a pixel, reflected (beam 22)with the same or the opposite handedness. After reflection on the pixel,the circularly polarized light of the opposite handedness is convertedin a phase plate or retarder 24 into linearly polarized light andreaches a polarizer 25 with such a direction of the transmission axis inthis embodiment that the reflected light is absorbed. Similarly,circularly polarized light of the same handedness is passed by thepolarizer 25.

[0030] Stray light, which is reflected on internal surfaces (forexample, the surface 16), has a handedness which is opposed to that ofthe beam 22 and is also converted by the retarder 24 into linearlypolarized light which is absorbed by the polarizer 25 (beams 26). Alsoparasitic light generated in the optical waveguide 18 due to internalreflection is absorbed by the polarizer 25 (beam 27).

[0031]FIG. 2 is a cross-section of a first embodiment of an opticalwaveguide with which the above-mentioned effect can be achieved. On anexit face 19, the optical waveguide 18 has a plurality of grooves 30which are filled with a chiral nematic liquid crystalline mixture andare covered with a 20-50 μm thick plate 31 of, for example, acryl orglass. On the side facing the light source, the grooves 30 preferablyextend at an angle of 45 degrees to the surface 19 so that light 21coupled out by the grooves leaves the optical waveguide substantiallyperpendicularly to the surface 16 in the direction of the display device2. Consequently, a very efficient illumination of the reflective displaydevice 2 is achieved. Since the grooves are filled with chiral nematicliquid crystalline material, levorotatory or dextrorotatory circularlypolarized light is reflected (beams 21), dependent on the material usedand on the surface treatment. In this embodiment, levorotatory light isreflected in a spectral range determined by the pitch p of the chiralnematic liquid crystalline material and the refractive indices n_(e),n_(o) (n_(e): extraordinary refractive index and n_(o): ordinaryrefractive index); light having a wavelength in the range betweenλ_(e)=n_(e)-p and λ_(o)=n_(o).p is reflected. Dextrorotatory polarizedlight 20′ remains within the optical waveguide 19 due to reflection onthe surfaces 16, 19 and due to a favorably chosen angle of incidence ofthe beam 20, and, after internal reflections can again be reflected on agroove 30.

[0032] In the embodiment of FIG. 3, the grooves 30 are provided withchiral nematic polymer networks. The pitch of the chiral nematicmaterial in each groove 30R, 30G, 30B is adapted in such a way that red(beam 21R), blue (beam 21B) and green (beam 21G) light is reflected andleaves the optical waveguide substantially perpendicularly to thesurface 16 in the direction of the display device 2. The referencenumerals again denote the same components as those in FIG. 2. In thisway, different grooves couple different parts of the spectrum, with avery good adaptation being possible to the wavelength of the lightsource(s) 12, notably when LEDs having a narrow emission spectrum areused for this purpose. When the choice of the liquid crystal materialand the pitch limits the reflection band to a very narrow band (at mostequal to that of the spectrum emitted by the LED) the light to bereflected and the reflected light are minimally disturbed during use inreflection (when the light source 12 is switched off).

[0033] The pitch of the chiral nematic material within a groove 30 mayalso vary to such an extent that a wide spectrum is reflected so thateach groove 30 reflects beams 21R, 21G and 21B (FIG. 4).

[0034] The mutual parts (display device, optical waveguide andretarder-polarizer combination) are preferably mutually secured by meansof a transparent adhesive having a low refractive index. The choice of alow refractive index also prevents the above-mentioned parasiticreflections.

[0035] FIGS. 5 to 7 show the method of manufacturing an opticalwaveguide which does not have microgrooves but generates circularlypolarized light which is deflected towards the reflective image displaypanel 2. A thin layer 33 of a chiral nematic liquid crystal polymermaterial is provided on a basic substrate 32 of, for example, glass andis coated, if necessary, with an isotropic transparent protectivecoating 34. The coating 34 is subsequently made locally isotropic, inthis case by means of laser beams 35 which are incident at an angle of45 degrees. The chiral nematic liquid crystal polymer material remainsanisotropic in the areas which are not irradiated. By suitable choice ofthe ratio between isotropic and anisotropic areas, an optical waveguide18 is obtained with areas 30 which convert an incident light beam intolight beams 21 leaving the optical waveguide in the direction of thedisplay device 2.

[0036] In the method shown in FIG. 6, the substrate is coated with amixture of chiral nematic monomers, which mixture is subsequentlyexposed via the mask 36 (by means of, for example UV radiation 37 againincident at an angle of 45 degrees) up to a temperature below theisotropic transition temperature. The chiral nematic ordering is therebylocally frozen (areas 30). Subsequently, the assembly is heated to atemperature above the isotropic transition temperature (by means of, forexample thermal radiation) so that the unexposed parts 39 becomeisotropic and are fixed to a polymer network by means of localillumination 38 (flood exposure).

[0037] Finally, the method shown in FIG. 7 makes use of“photo-isomerizable” chiral nematic polymers, a layer of which isprovided again between a basic substrate and a coating. The material ischosen to be such (pitch, refractive indices) that it reflects thedesired wavelength. By local UV illumination via the mask 36, the valueof the reflected wavelength shifts to higher values, for example, toinfrared. The unexposed parts 30 continue reflecting the desiredwavelength, while the other reflection is not visible to the human eye.

[0038] The protective scope of the invention is not limited to theembodiments described. It has already been noted that ellipticallypolarized light may be used alternatively, although this is at theexpense of the suppression of stray light. Also, other electro-opticaleffects may be used, for example, electrochromic effects. As mentionedin the opening paragraph a display comprising deformable mirrors may beused as well. Circularly polarized light may also be obtained in theoptical waveguide by providing a (pattern of) ¼λ plate(s), combined with(a) linear reflector and/or mirror(s). The invention resides in each andevery novel characteristic feature and each and every combination ofcharacteristic features. Reference numerals in the claims do not limittheir protective scope. Use of the verb “comprise” and its conjugationsdoes not exclude the presence of elements other than those mentioned inthe claims. Use of the article “a” or “an” preceding an element does notexclude the presence of a plurality of such elements.

1. A display device comprising an image display panel having a firstsubstrate which is provided with light-reflecting electrodes at the areaof pixels, an illumination system comprising an optical waveguide of anoptically transparent material having an exit face facing the imagedisplay panel and a plurality of end faces, at least one of said endfaces being an entrance face for light, while light can be coupled intosaid end face of the optical waveguide, characterized in that theoptical waveguide is present between the image display panel and acircular polarizer, and the optical waveguide comprises polarizing meansfor substantially circularly polarizing the entering light.
 2. A displaydevice as claimed in claim 1 , characterized in that the image displaypanel comprises a second light-transmissive substrate andelectro-optical material between the two substrates.
 3. A display deviceas claimed in claim 1 , characterized in that the polarizing meanscomprise a chiral nematic liquid crystal material.
 4. A display deviceas claimed in claim 1 , characterized in that the polarizing meanscomprise a chiral nematic polymer network.
 5. A display device asclaimed in claim 3 or 4 , characterized in that the polarizing means arepresent in grooves in the exit face of the optical waveguide facing theimage display panel.
 6. A display device as claimed in claim 5 ,characterized in that the pitch of the chiral nematic liquid crystalmaterial or the chiral nematic polymer network within a groove varies.7. A display device as claimed in claim 3 or 4 , characterized in thatthe polarizing means comprise a patterned chiral nematic polymer networkin the optical waveguide.
 8. A front light comprising an opticalwaveguide of optically transparent material and a plurality of endfaces, at least one of said end faces being an entrance face for light,while light can be coupled into said end face of the optical waveguide,characterized in that the optical waveguide comprises polarizing meansfor circularly polarizing the entering light.
 9. A front light asclaimed in claim 8 , characterized in that the polarizing means comprisea chiral nematic liquid crystal material.
 10. A front light as claimedin claim 8 , characterized in that the polarizing means comprise achiral nematic polymer network.
 11. A front light as claimed in claim 9or 10 , characterized in that the polarizing means are present ingrooves in the exit face of the optical waveguide.
 12. A front light asclaimed in claim 11 , characterized in that the pitch of the chiralnematic liquid crystal material or the chiral nematic polymer networkwithin a groove varies.
 13. A front light as claimed in claim 9 or 10 ,characterized in that the polarizing means comprise a patterned chiralnematic polymer network in the optical waveguide.
 14. A method ofmanufacturing a front light comprising polarizing means for circularlypolarizing the entering light, characterized in that a surface of atransparent body is provided with grooves, and the transparent bodywithin the grooves is provided with a chiral nematic liquid crystalmaterial or the chiral nematic polymer network.
 15. A method ofmanufacturing a front light comprising polarizing means for circularlypolarizing the entering light, characterized in that a surface of atransparent body is provided with a layer of a chiral nematic materialwhich is locally converted into isotropic material.
 16. A method ofmanufacturing a front light comprising polarizing means for circularlypolarizing the entering light, characterized in that a surface of atransparent body is provided with a layer of a chiral nematic materialwhich is locally converted into chiral nematic material having areflection band outside the visible range.